The invention relates to wireless speakerphones, and more particularly, to a microprocessor-controlled full-duplex speakerphone using automatic gain control. There are two basic types of speakerphones available on the market today: A lower-cost, half-duplex design aimed at the consumer market, and an expensive full-duplex DSP implementation for business applications. The major technical obstacle to overcome in designing a speakerphone is the prevention of unstable feedback (howling, or squealing) caused by adjusting the speaker and/or microphone gains too high. The first solution to this problem was the half-duplex speakerphone.
In the receive direction (the far-end person is heard via the speaker), it is obviously desirable to provide a relatively large gain on the speaker, but due to the proximity of the microphone to the speaker in the speakerphone enclosure, the microphone will detect the far-end person's voice and amplify it back to the far-end. This acoustic coupling is the source of half of the feedback loop in the speakerphone, and results in an annoyingly high level of sidetone in the far-end handset. To mitigate this acoustic coupling, the half-duplex speakerphone reduces the gain of the microphone to its minimum when the far-end person is talking, so that none of the far-end person's voice is returned back.
In the transmit direction (the near-end person speaks into the microphone), it is obviously desirable to provide a relatively large gain on the microphone to allow greater distances between the person speaking and the microphone. However, due to the electrical connection of the microphone to the telephone lines (via the 2-wire to 4-wire hybrid interface), a part of the transmitted voice signal is reflected back into the RX speech path, which is then amplified by the speaker driver with the result that the near-end person's own voice is amplified into the room. This hybrid sidetone is the second half of the feedback loop in the speakerphone, and is the natural way typical telephone handsets provide sidetone from microphone to the earpiece (the microphone gain contributes to the sidetone level). To mitigate this hybrid sidetone, the half-duplex speakerphone reduces the gain of the speaker to its minimum when the near-end person is talking, so that none of the near-end person's voice is amplified into the same room.
Whenever the microphone and speaker gains are not balanced in this “see-saw” minimum/maximum way, the familiar acoustic feedback sound (howling, squealing) can easily result from the completed feedback loop provided by the acoustic coupling and hybrid sidetone audio paths. This “see-saw” gain adjustment process requires the speakerphone to determine which person is talking, and it must arbitrate the two signal paths accordingly. These functions are typically provided by an expensive analog voice-switched speakerphone chip, but the arbitration typically suffers from several basic disadvantages:
The speakerphone gives priority to the loudest person speaking when both people are attempting to speak simultaneously, for example, when one person is trying to interrupt the other. This is a disadvantage for the case of a weak signal from a distant phone, or from a person who is not sitting very close to the speakerphone. In these cases it may be necessary for the far-end person to unnaturally shout into the handset, or for a person at the far end of the table to temporarily move closer to the speakerphone.
The slow switching time during this volume comparison usually results in the loss of a few syllables at the beginning of the interruption, which generally results in the person having to repeat the whole sentence.
The full-duplex DSP-based speakerphone implements robust signal cancellation of the two speech paths to eliminate the coupling of the two channels. As a result, the microphone and speaker gains can be maintained at high levels throughout the conversation, thus eliminating the voice switching altogether. However, this high quality demands a high price because 1) it requires a powerful DSP engine capable of performing these calculations on both speech paths at the 8 kHz sample rate, and 2) the algorithm requires considerable DSP and audio experience.
The user-controlled volume setting is typically implemented in hardware in an analog speakerphone via a potentiometer, but suffers from degraded audio quality over time due to dust in the potentiometer mechanism and DC offset drift. Typical cordless phone designs house the speakerphone function in the base station. The disadvantage of this configuration is the obvious one: the speakerphone functions are limited by the length of the wires connecting the base station to the telephone jack and to the power.
In view of the foregoing, what is needed is a speakerphone which allows a full-duplex conversation, i.e.: simultaneous speaking and hearing, without an external analog speakerphone chip and a DSP engine, without external analog decoders with resistor ladders for providing gain, and without using a potentiometer for user-controlled volume control. Furthermore, the speakerphone functions should be housed in the portable handset.